In the production of crude oil from an offshore oil field there is carried out a separation of the well stream into water, oil and natural gas. The natural gas following the produced crude oil in the well stream, and which is commonly designated "associated gas", will often be desired to be transported from the offshore field to a receiving system on land.
An obvious thought has been to build a plant for the production of liquefied associated gas at a production platform or a production ship which is installed on the field, and which receives the well stream for processing. After separation of the stream into water, oil and gas, the separated gas should be able to be condensed to a liquid condition in which it has a low pressure and a low temperature, and thereafter to be transferred through a pipeline system to a vessel for transport to land in this condition. However, this is not feasible in a practical manner with the technique of today, since cryogenic transfer of liquefied natural gas via conventional "loading arms", or even via more sophisticated transfer systems, is associated with hitherto unsolved problems with freezing, clogging of passages, etc. Such transfer is also associated with danger of an unintended spill of liquefied natural gas onto the sea, which might result in explosion-like evaporation, with a substantial destructing potential.
I order to avoid the cryogenic transfer of condensed gas from the production platform or the production ship to the transport vessel, one could instead think of transferring the natural gas from the production platform or ship to a transport vessel equipped with a complete conventional plant for liquefaction of the natural gas, for production of mainly LNG. However, conventional plants for the production of LNG are very expensive, and it is therefore not economically acceptable to build such plants on individual transport vessels.
Another proposal for solving the problem is described in U.S. Pat. No. 5,025,860. Here is described a system wherein a natural gas from an offshore petroleum deposit is carried to a production platform or production ship where carbon dioxide and water are separated from the natural gas, whereafter the natural gas is compressed and cooled to a compressed gas condition. In this gas condition, under a high pressure, the natural gas is thereafter transported through a pipeline system to an LNG tanker where it is expanded and cooled to form of liquefied natural gas (LNG) which is stored on board in the tanks of the ship. In the expansion and cooling process on board the LNG tanker there is also obtained a non-condensed residual gas which can be carried back to the production platform or ship.
Advantages which are achieved with this natural gas processing system according to U.S. Pat. No. 5,025,860 are stated to be that practically all of the energy required for liquefying the gas on board the LNG-tanker is supplied to the gas on board the production platform or ship. Consequently, the investment cost of the necessary and expensive installation for supply of this energy to the gas will be connected to the production platform or ship, whereas every single one of the LNG-tankers which is to transport the natural gas, only needs a relatively limited production equipment on board, which in addition can be mounted on the upper deck of the ship, on replaceable frame structures suitable for-the purpose.
The natural gas which is of current interest to be conditioned for transport by means of this system according to U.S. Pat. No. 5 025 860, may come from a natural gas source, or it may be a by-product from an oil source (associated gas).
That part of the natural gas processing system according to U.S. Pat. No. 5,025,860 which is installed on board the production platform or ship, and which has for its purpose to purify the natural gas and thereafter to compress and cool the gas for delivery to the LNG-tanker in compressed gas condition, is designed as a traditional plant for this purpose.
That part of the natural gas processing system according to U.S. Pat. No. 5,025,860 which is installed on board the LNG tanker, comprises an expansion plant, wherein the received, cooled high-pressure gas is subjected to an additional cooling and is expanded adiabatically in three stages. A liquefied LNG gas with a pressure of ca. 1 bar is transported as a final product from the expansion plant to storage tanks on board, prepared for transport. Non-condensed gas from the expansion plant is carried through a compression group where it is compressed to a pressure of ca. 30 bar, whereafter it is returned to the production platform or ship through a return line, for use e.g. as a fuel for operation for the compressors for compression and cooling of the natural gas on board the production platform or ship.